Update youtube-dl to 2018.12.9

[pyup-bot]

This PR updates youtube-dl from 2018.12.3 to 2018.12.9.

Changelog

2018.12.09

Core
* [YoutubeDL] Keep session cookies in cookie file between runs
* [YoutubeDL] Recognize session cookies with expired set to 0 (12929)

Extractors
+ [teachable] Add support for teachable platform sites (5451, 18150, 18272)
+ [aenetworks] Add support for historyvault.com (18460)
* [imgur] Improve gallery and album detection and extraction (9133, 16577,
17223, 18404)
* [iprima] Relax URL regular expression (18453)
* [hotstar] Fix video data extraction (18386)
* [ard:mediathek] Fix title and description extraction (18349, 18371)
* [xvideos] Switch to HTTPS (18422, 18427)
+ [lecturio] Add support for lecturio.com (18405)
+ [nrktv:series] Add support for extra materials
* [nrktv:season,series] Fix extraction (17159, 17258)
* [nrktv] Relax URL regular expression (18304, 18387)
* [yourporn] Fix extraction (18424, 18425)
* [tbs] Fix info extraction (18403)
+ [gamespot] Add support for review URLs

Links

• PyPI: https://pypi.org/project/youtube-dl
• Changelog: https://pyup.io/changelogs/youtube-dl/
• Repo: https://github.com/rg3/youtube-dl

Resolves #500